Highly integrated fuel processor for distributed hydrogen production

ABSTRACT

A recuperative heat exchanger ( 36 ) is provided for use in a fuel processor ( 20 ), the heat exchanger ( 36 ) transferring heat from a fluid flow ( 34 ) at one stage of a fuel processing operation to the fluid flow ( 32 ) at another stage of the fuel processing operation. The heat exchanger ( 36 ) includes a housing ( 56 ) defining first and second axially extending, concentric annular passages in heat transfer relation to each other; a first convoluted fin ( 70 ) located in the first passage to direct the fluid flow therethrough; and a second convoluted fin ( 72 ) located in the second passage to direct the fluid flow therethrough.

FIELD OF THE INVENTION

This invention relates to fuel processors, and in more particularapplications to fuel processors for distributed hydrogen production.

BACKGROUND OF THE INVENTION

It has been well-established that a critical key to the long-termsuccess of fuel cell vehicles is the development of a hydrogeninfrastructure. Fuel cell vehicles are projected by many to be theeventual replacement of, or at the very least supplement to, theinternal combustion engine vehicle. This is driven primarily by thegrowing concerns over greenhouse gas and air pollutant emissions,long-term availability of fossil fuels, and energy supply security. Theproton exchange membrane (PEM) fuel cells which are the focus of almostall the current efforts towards the development of commercially viablefuel cell vehicles require hydrogen as a fuel. Virtually all effortstowards the on-board production of hydrogen from more portablehydrocarbon fuels have been abandoned in recent years, and almost allfuel cell vehicle manufacturers are currently focusing on refueling thevehicles with high-purity liquid or gaseous hydrogen.

The means by which hydrogen can be produced in large quantities are wellunderstood. Steam reforming of methane is the primary means by whichhydrogen is currently being produced on an industrial scale. Today,about half of the world production of hydrogen is used in oilrefineries, mainly for the production of automotive fuels. Another 40%is consumed in the commercial production of ammonia. However, the annualproduction volume of hydrogen in the United States is comparable to onlytwo days worth of gasoline consumption. Furthermore, hydrogen iscurrently being produced predominantly on a large industrial scale. Fora successful transportation infrastructure, the hydrogen refuelingnetwork must be well-distributed. Hydrogen is, however, very problematicto distribute. Gaseous hydrogen has one of the lowest energy densities,making it difficult to transport in the amounts that would be requiredfor a transportation fuel cell infrastructure. Distributing hydrogen inliquid form is also difficult—it requires very low temperatures (22K),and even in liquid form hydrogen has a low energy density. Because ofthese concerns, it can be reasonably concluded that a hydrogeninfrastructure capable of supplying the refueling needs of fuel cellvehicles will need to rely on the distributed production of high-purityhydrogen.

The widely distributed hydrogen production necessary for atransportation fuel cell infrastructure is much smaller than the typicalrefinery or ammonia-producing hydrogen production scale. Various meansby which high-purity hydrogen can be economically produced at this smallscale are currently being pursued. One such means of production is toapply, on a smaller scale, the well-understood methods of producing ahydrogen at the current large scales. The predominant method ofproducing a hydrogen at large scales is by steam reforming natural gas(methane) over a catalyst. The steam reforming reaction produceshydrogen and carbon monoxide as follows:CH₄+H₂O→3H₂+CO

The steam reforming reaction is highly endothermic, requiring 206 kJ ofenergy per mole of methane consumed. Some of the CO produced isconverted to CO₂ via the associated water-gas shift reaction:CO+H₂O→CO₂+H₂

This reaction is exothermic, and liberates 41 kJ of energy per mole ofCO consumed. Steam reforming of methane is typically carried out attemperatures in the range of 700° C.-900° C. Since the reaction isendothermic, heat must be supplied to the reactor. This is typicallyaccomplished by loading the catalyst into a series of tubes which areplaced in a furnace. The hydrogen can be extracted from the steamreforming product gas (reformate) through various well-understood means,such as metal membrane or pressure swing adsorption (PSA).

It has long been understood that in order to make steam reforming ofnatural gas feasible at the smaller scales required for a distributedproduction of hydrogen for fuel cell vehicles, a greater integrationbetween the heat-producing combustor and the endothermic steam reformingreaction is needed. Attempts to build such systems have met with somesuccess in the past, but the performance efficiency has always beenlimited by the ability to transfer the required heat into the steamreforming reaction without generating extremely high (>1000° C.) metaltemperatures.

SUMMARY OF THE INVENTION

Embodiments of the invention are disclosed herein in a highly integratedsteam reforming fuel processor, which in combination with a PressureSwing Adsorption (PSA) can deliver high purity hydrogen at a scale whichis well-suited to the distributed production of hydrogen for atransportation fuel cell infrastructure. This fuel processor overcomesthe heat transfer limitations of previous designs, and is therebycapable of achieving a high level of hydrogen product efficiency withoutextremely high metal temperatures.

In accordance with one feature of the invention, a recuperative heatexchanger is provided for use in a fuel processor, the heat exchangertransferring heat from a fluid flow at one stage of a fuel processingoperation to the fluid flow at another stage of the fuel processingoperation.

According to one feature of the invention, a fuel processing unitincludes a steam reformer and a recuperative heat exchanger, the heatexchanger connected to the steam reformer to direct a fluid flow to thesteam reformer, the fluid flow being a steam/fuel feed mix to bereformed in the steam reformer into a reformate, the steam reformerconnected to the heat exchanger to direct the fluid flow back to theheat exchanger after it has been reformed into the reformate.

In accordance with one feature, the heat exchanger includes a housingdefining first and second axially extending, concentric annular passagesin heat transfer relation to each other; a first convoluted fin locatedin the first passage to direct the fluid flow therethrough; and a secondconvoluted fin located in the second passage to direct the fluid flowtherethrough.

In one feature, the recuperative heat exchanger further includes acylindrical water-gas shift reactor extending centrally through thehousing at a location radially inward from the first and secondpassages.

In accordance with one feature of the invention, the heat exchangerincludes a cylindrical wall; a first convoluted fin bonded to a radiallyinwardly facing surface of the wall, the convolutions of the firstconvoluted fin extending axially to direct the fluid flow through theheat exchanger; and a second convoluted fin bonded to a radiallyoutwardly facing surface of the wall, the convolutions of the secondconvoluted fin extending axially to direct the fluid flow through theheat exchanger.

Other objects, features, and advantages of the invention will becomeapparent from a review of the entire specification, including theappended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a fuel processing systemembodying the invention;

FIG. 2 is a perspective view of an integrated steam reformer/combustorassembly of the invention;

FIG. 3 is a section view of one embodiment of an integrated fuelprocessing unit of the invention;

FIG. 4 is a flow schematic showing the fluid flows through the fuelprocessing unit of FIG. 3;

FIG. 5 is a sectioned, perspective view from above of the integratedfuel processing unit of FIG. 3;

FIG. 6 is a temperature versus flow path graph showing the temperatureprofile for one embodiment of the steam reformer/combustor of FIG. 2;

FIG. 7 is a sectioned, perspective view from above of another embodimentof an integrated fuel processing unit of the invention;

FIG. 8 is an enlarged section view of the portion encircled by line 8-8in FIG. 7 and highlighting selected components of the integrated fuelprocessing unit;

FIG. 9 is an enlarged section view of the portion encircled by line 9-9in FIG. 7 and highlighting selected components of the integrated fuelprocessing unit;

FIG. 10 is an enlarged section view of the portion encircled by line10-10 in FIG. 7 and highlighting selected components of the integratedfuel processing unit;

FIG. 11 is a section view taken from line 11-11 in FIG. 10;

FIG. 12 is a view similar to FIG. 10, but highlighting other componentsof the integrated fuel processing unit;

FIG. 13 is a view similar to FIGS. 10 and 12, but again highlighting yetother components of the integrated fuel processing unit;

FIG. 14 is an enlarged view of the portion encircled by line 14-14 inFIG. 7;

FIG. 15 is a view taken from line 15-15 in FIG. 14; and

FIG. 16 is a view similar to FIG. 8, but highlighting other componentsof the fuel processing unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A system schematic of a highly integrated fuel processor 20 is shown inFIG. 1. In this system, the only source of fuel required for thecombustor is the hydrogen-depleted off gas 21 from the Pressure SwingAbsorption (PSA) 22. The high-pressure side of the PSA 22 is preferablydesigned to operate at 100 psig, while the low-pressure side of the PSAis designed to operate at near-atmospheric pressure (˜1 psig). Heatrecovered from a combustor exhaust 24 of combustor 25 is used in avaporizer 29 to vaporize and superheat a water feed 26 for a steamreformer 28, as well as to preheat a natural gas feed 30 for the steamreformer 28 in a fuel preheater 31. Heat is recovered from a reformatestream 32 exiting the steam reformer 28 and is used to further preheatthe now mixed steam-natural gas feed 34 in a recuperative heat exchanger36. A water-gas shift (WGS) reactor 38 is used to increase hydrogenproduction. Downstream of the water-gas shift reactor 38, additionalheat is recovered from the reformate stream 32 and is used to preheat acombustor feed 40 and the water feed 26 to the vaporizer 29. Thereformate stream 32 is cooled to a temperature suitable for the PSA 22,and excess water is condensed out. The heat removed in this process isat a fairly low temperature, and is not recovered. The condensate 41 canbe recovered and reused, if it is desirable to do so.

Optimized hydrogen conversion is accomplished in this design through theuse of an integrated steam reformer and combustor 42 which integratesthe reformer 28 and the combustor 25 and which has highly effective heattransfer characteristics. The integrated steam reformer and combustor 42will hereafter in this description be referred to as the SMR reactor 42.As best seen in FIG. 2, the SMR reactor 42 is constructed as a metalcylinder 44 of a high-temperature alloy with a first convoluted finstructure 46 brazed to an inner surface 48 along the entirecircumference of the cylinder 44, and a second convoluted fin structure50 brazed to an outer surface 52 along the entire circumference of thecylinder 44. The inside fin 46 is wash-coated with a steam reformingcatalyst, and the outside fin 50 is wash-coated with a catalyst capableof oxidizing both hydrogen and methane. The cylinder 44 is of sufficientthickness to be used as a portion of a pressure vessel 54, shown in FIG.3, which contains the high-pressure steam reformer feed 34 and reformatestream 32, shown in FIG. 4. Cylindrical metal sleeves 56 and 58 (notshown in FIG. 2) serve to channel the flow through the coated finstructures 46 and 50, respectively.

FIGS. 3 and 4 show one embodiment of the fully assembled fuel processor20. The fuel processor 20 consists of the cylindrical high-pressurevessel 54 situated within and coaxial to a low-pressure cylinder orvessel 60. The water and natural gas feeds 26,30 for the steam reformer28 are vaporized (in the case of the water) and preheated in a coiledtube 62 which is situated between the two cylinders 44,60. The preheatedfeed 34 enters the pressure vessel 54 through a tube 64 at a top domedhead 66 of the vessel 54 and passes into the recuperator 36.

The construction of the recuperator 36 is similar to the SMR reactor 42,with a highly augmented fin structures 70 and 72 (such as a louvered orlanced-offset fin) brazed to both the outside and inside surfaces 76 and78 of the cylinder 56. The flows will again be channeled through thesefin structures 70,72 by cylindrical metal sleeves 44 and 80. Therecuperator cylinder 56 and fins 70,72 are sized such that the annulararea encompassing the fin 70 bonded to the outer surface 76 is the sameas the annular area encompassing the steam reforming catalyst coated fin46 in the SMR reactor 42. The recuperator cylinder 56 extends past theends of the fins 70,72 on one side by an amount approximately equal tothe length of the SMR reactor 42. This allows the recuperator cylinder56 to function as the previously mentioned inner sleeve 56 for the SMRreactor 42. Similarly, the SMR reactor cylinder 44 can extend past itsfins 46,48 so that it functions as the outer sleeve 44 for therecuperator 36.

In the fully assembled fuel processor 20, the steam reformer feed 34flows through the outer fin 70 of the recuperator 36, then passesthrough the inner (steam reforming catalyst coated) fin 46 of the SMRreactor 42, where it is converted to a hydrogen rich reformate 32. Thereformate flow 32 is then baffled so that upon exiting the steamreforming fin 46 it turns upward and passes along the inner surface 78of the extended recuperator cylinder 56 and flows up through the innerfin 72 of the recuperator 36, where it transfers heat to the incomingsteam reformer feed 34.

The water-gas shift (WGS) reactor 38 is a cylindrical catalyst-coatedmonolith 84 which is encased in insulation 86 and is located within theinner sleeve 80 of the recuperator 36. The reformate flow 32 exits therecuperator 36 towards the top of the pressure vessel 54, where it isforced to reverse direction due to a domed head 88 which separates thereformate 32 from the preheated steam reformer feed 34 entering thepressure vessel 54. The reformate 32 flows down through the WGS monolith84 at the center of the cylinders 80 and 90. Upon exiting the WGSreactor 38, the reformate flow 32 is diverted towards the walls 44,92 ofthe pressure vessel 54 and passes through a combustor preheater 94.

The construction of the combustor preheater 94 is very similar to thatof the SMR reactor 42 and the recuperator 36, with highly augmented finstructures 96,98 (such as a louvered fin) brazed to both the outside andinside surfaces 100,102 of the cylinder 92. As was the case with thereactor 42, the cylinder 92 serves as a part of the pressure vessel 54and is welded to the SMR reactor cylinder 44. The reformate 32 passesthrough the fin 98 on the inside surface 102 of the preheater 94, andtransfers heat to the combustor feed gases 40 which pass through the fin96 on the outer surface 100 of the preheater 94 in a countercurrentdirection. Upon exiting the fin 96, the reformate 32 passes over a waterpreheater 104, which consists of a coiled tube 106 through which thewater 26 for the steam reformer 28 is flowing. It is expected that thereformate 32 is cooled in these preheaters 94, 104 to such an extentthat some water is condensed out of the reformate 82. Downstream of thewater preheater 104, the reformate 32 (and any condensate) reach thebottom dome 108 of the pressure vessel 54 and exit the vessel 54 to passto a heat exchanger 110 which cools the reformate 32 down to atemperature appropriate for the PSA 22. The heat removed from thereformate 32 in this heat exchanger 110 is considered to be waste heat,and can be discharged to the surrounding ambient.

The hydrogen-depleted off gas 21 from the PSA, now at near-atmosphericpressure (˜1 psig), is mixed with the combustor air 112 to comprise thecombustor feed 40. This feed gas 40 passes into the low-pressurecylinder 60 and flows up through the combustor preheater 94 and into thefin 50 on the outer surface 52 of the SMR reactor 42. The combustor feed40 flows vertically up though this catalyst-coated fin 50,counter-current to the flow 32 passing through the fin 46 on the innersurface 48 of the SMR reactor 42. The hydrogen, methane, and carbonmonoxide in the combustor feed 40 are catalytically combusted as theflow passes through the fin 50. The heat generated is conducted throughthe cylindrical wall 44 of the SMR reactor 42 and feeds the endothermicsteam reforming reaction occurring on the fin 46 attached to the insidesurface 48 of the SMR reactor 42.

Upon exiting the fin 50 of the reactor 42, the combustor exhaust gas 24continues to flow upward through the annular region between thelow-pressure and high-pressure cylinders 44,60, passing over the watervaporizer 29 and natural gas preheater 31. The water vaporizer 29 andnatural gas preheater 31 consist of the coiled tube 62 which resideswithin the annular space between the cylinders 44,60. The preheatedliquid water 26 enters the coiled tube 62 at the bottom and flowsupward, receiving heat from the high-temperature combustor exhaust 24which is flowing over the tube 62. As the water passes through the tube62, it is fully vaporized and then mildly superheated. The natural gas30 enters the coiled tube 120 at some point along the length of the coil62, and mixes with the superheated steam. Both fluids are then furtherheated by the combustor exhaust in the remaining length of the coiledtube 62, after which they are piped into the high-pressure vessel 54. Analternative design (best seen in FIGS. 7 and 9) would have the naturalgas 30 preheated in a separate coiled tube downstream (with respect tothe combustor exhaust flow 24) of the water coil, and the two fluidswould be mixed after exiting their respective coiled heat exchangers.

A large percentage of the combustion reaction typically occurs over arelatively small initial length of the catalyst region. It can beadvantageous to force the combustion reaction to be more evenlydistributed over the length of the reactor 42. Since the combustionreaction is diffusion-limited, this can be achieved to some extent byhaving an initial region where the convoluted fin structure 50 isuninterrupted, thus providing a more laminar flow which minimizesdiffusion, and an exit region in which the convoluted fin structure 50is turbulated by the use of louvers, slits, lances, etc. to promotegreater diffusion of the reactants for final cleanup of the methane,hydrogen, and carbon monoxide.

FIG. 6 illustrates the expected typical temperature profile within theSMR reactor 42. The SMR feed 32,34 is flowing from right to left, whilethe combustor feed 40 flows from left to right. The temperatures at theextreme tips of the fins 46 and 50, as well as the temperature on eithersurface of the cylinder wall 44, are also depicted. It can be seen inthe graph that the heat transfer within the SMR reactor 42 is sufficientto keep the metal temperatures well below 1000° C. The ability to runthe SMR flow 32,34 and the combustor flow 50 in a countercurrentdirection without causing dangerously high metal temperatures results ina reformate exit temperature which is substantially higher than thereformate inlet temperature, thus maximizing the conversion of methaneto hydrogen and minimizing the required effectiveness of the recuperator36.

Since this design avoids the need for additional natural gas to besupplied to the combustor 25, the control of the fuel processor 20 canbe greatly simplified. Temperatures within the SMR reactor 42 can becontrolled by adjusting the combustor air flow 112, based on temperaturefeedback provided by a sensor (not shown) located on the outer sleeve 58of the reactor 42 in the area where the peak exhaust gas temperature isexpected. Further control is possible by incorporating an adjustablewater bypass valve (not shown) of the water preheater 104, so that thetemperature of the water 26 being supplied to the vaporizer 29 can beadjusted by varying the percentage of water flow through the preheater104. Feedback from a temperature sensor located at the inlet 122 to theWGS reactor 38 could potentially be used as the control source for thisvalve.

The high degree of thermal integration results in a volumetricallycompact high-pressure vessel 54, thus minimizing the wall thicknessrequired for a vessel operating at the elevated temperatures andpressure required for the application. One preferred embodiment of thefuel processor 20 described in this application has a pressure vessel 54which is 6 inches in diameter, with a total length of approximately 40inches, and is expected to be capable of reforming 6.25 kg/hr of naturalgas with a hydrogen production efficiency of 77.5% (the LHV of thehydrogen removed by the PSA, assuming 75% of the hydrogen in thereformate is removed, divided by the LHV of the natural gas feed),resulting in a hydrogen production rate of 1.87 kg/hr.

Another embodiment of the fuel processing unit 20 is shown in FIGS.7-16. This embodiment differs from that of FIGS. 2-4 in that:

-   -   (a) the water preheater 104 has been moved from the integrated        unit 20 to an external location and the combustor preheater 94        extends over the region previously occupied by the water        preheater 104;    -   (b) the PSA off gas inlet and the reformate outlet, together        with the region associated therewith, have been modified; and    -   (c) a separate coiled tube 130 for the natural gas preheater 31        has been added downstream from the coiled tube 62 for the water        vaporizer 29.

As in the fuel processing unit 20 of FIG. 3, the fins 46,50 of the SMRreactor 42 are brazed to the cylinder 44 only and the fins 96,98 of thecombustor preheater 94 are brazed to the cylinder 92 only, with thecylinders 44 and 92 being welded at their adjacent ends to form thecylindrical wall of the high-pressure vessel 54. Further, as with theembodiment of the fuel processing unit 20 of FIG. 3, a cylindricalbaffle or wall 132 is used to extend the inner boundary of the flow pathfor the combustor exhaust 24 through the portion of the coils 62,130that extend beyond the top 66 of the pressure vessel 54. Thiscylindrical baffle 132 is tack welded to the top 66 of the pressurevessel 54, but does not contact the coils 62,130 or the low pressurevessel 60. Accordingly, the high-pressure vessel 54 and the low-pressurevessel 60 are mechanically coupled in only two locations. The first,best seen in FIG. 8, is near the bottom of the pressure vessel 54 wherean Air/PSA off-gas inlet structure 134 is welded to both the lowerdome-shaped head 108 for the high-pressure vessel 54 and a lowerdome-shaped head 136 for the low-pressure vessel 60 to form a rigidconnection. The second location is near the top of the fuel processingunit 20, as best seen in FIG. 9, where the coils 62,130 are welded atfirst locations 138 to the low-pressure vessel 60 and at a secondlocation 140 to the dome-shaped top head 66 of the high pressure vessel54 via a feed mix inlet tube structure 142. The structure 142 includes afeed mix inlet manifold 143 that is connected to respective outlet ends144 and 145 of the coils 62 and 130 by suitable fitting connections, anda downwardly extending mixing tube 146 with an internal mixing structure147 (also shown in the illustrated embodiment is a centralinstrumentation tube 148 which may optionally be eliminated forproduction units). This second connection is far from rigid, being madethrough what are in effect two large springs (the coils 62,130).Accordingly, the high-pressure vessel 54 and the low-pressure vessel 60are largely unconstrained in the axial direction relative to each other,and although the outer shell 58 of the vessel 60 will tend to runhotter, the differential thermal expansion should not generatesignificant stress.

As with the embodiment of the fuel processing unit 20 of FIG. 3, thefins 70, 72 of the recuperator 36 are brazed to the recuperator cylinder56 only, not to the high-pressure vessel 54 or the adjacent inboardsurfaces. As best seen in FIG. 10, the high-pressure vessel 54 andrecuperator cylinder 56 are mechanically coupled at the top end only,where both are welded to a feed mix distribution ring 150 that serves tomount the dome head 88 and cylinder 56 to the high-pressure vessel 54.As best seen in FIG. 11 the feed mix distribution ring 150 includes aplurality of angularly spaced holes 152 which allow the steam reformerfeed 34 to pass through to the recuperator 36 and the SMR reactor 42.Because the components 54,56 are connected at only one end, they arefree to move independently in response to differential thermalexpansion, and accordingly should not generate significant stress as aresult thereof.

Preferably, the inboard cylinder 80 does not contact the fins 70, 72 orthe cylinder 78, but rather is connected to the inside of the cylinder44 of the pressure vessel 54 via a flanged, ring-shaped baffle 154 thatis welded to both of the cylinders 44 and 80, as best seen in FIG. 12.Because the inboard cylinder 80 and the high-pressure vessel 54 are onlyconnected at one location, they are free to move independently of oneother in response to differential thermal expansion and therefore shouldnot generate significant stresses.

As with the embodiment of the fuel processor 20 of FIG. 3, thecylindrical wall 90 of the WGS reactor 38 is connected to the inboardcylinder 80 by a pair of flat, ring-shaped baffles 156, 158 welded toboth ends of the WGS cylinder 90, as best seen in FIG. 13. The baffle158 located at the bottom is also welded to the inside of the inboardcylinder 80. Because the annulus between the WGS cylinder 90 and theinboard cylinder 80 is not a flow channel, the annulus does not requirea gas-tight seal at both ends. Because the upper ring-shaped baffle 156is free to move relative to the inboard cylinder 80, the cylinders 80and 90 can move relative to each other in response to differentialthermal expansion and therefore should not generate significantstresses.

As with the embodiment of the fuel processing unit 20 of FIG. 3, thecentral volume of the combustor preheater region is occupied by acylindrical enclosure 160 that is preferably filled with insulation, asbest seen in FIG. 14. A castellated disc 162 is welded to the bottom ofthe enclosure 160 and to the inside of the cylinder 92 of the pressurevessel 54. As best seen in FIG. 15, angularly spaced notches 164 areprovided in the perimeter of the disc 162 in order to allow passage ofthe reformate flow 32. Preferably, the enclosure 160 is not attached tothe fins 98. Again, because the enclosure 160 and the pressure vessel154 are connected only at one end, they can move independently inresponse to differential thermal expansion and accordingly should notgenerate significant stresses as a result thereof.

As best seen in FIG. 16, the reformate flow 32 exits the fuel processingunit via a small diameter tube 166 welded to the lower head 108 of thepressure vessel 54. Combustion air 112 enters at the bottom of the unit20 via a large diameter tube 168 that is preferably concentric with thepressure vessel 54. A PSA offgas inlet structure 134 includes a tube 172of the same diameter as the tube 168, a PSA off gas inlet tube 174, anda combustor air flow/PSA offgas injector 176 which includes a pluralityof circumferentially spaced holes 178 that inject the PSA offgas intothe airflow 112. While the reformate outlet tube 166 and the offgasinlet structure 134 are rigidly connected to each other at twolocations, allowing for stresses to develop with differential thermalexpansion, there is little temperature difference between the PSA offgas flow 21 and the reformate flow 32. Accordingly, no significantstresses are to be expected.

1. A recuperative heat exchanger for use in a fuel processing system,the heat exchanger transferring heat from a fluid flow at one stage of afuel processing operation to said fluid flow at another stage of thefuel processing operation, the heat exchanger comprising: a housingdefining first and second axially extending, concentric annular passagesin heat transfer relation to each other; a first convoluted fin locatedin the first passage to direct the fluid flow therethrough and a secondconvoluted fin located in the second passage to direct the fluid flowtherethrough.
 2. The recuperative heat exchanger of claim 1 wherein saidhousing further defines a first inlet to direct the fluid flow into thefirst passage at one end of the first passage, a first outlet to directthe fluid flow from the first passage at an opposite end of the firstpassage, a second inlet to direct the fluid flow into the second passageat a location adjacent said opposite end, and a second outlet to directthe fluid flow from the second passage.
 3. The recuperative heatexchanger of claim 1 wherein said first passage is located radiallyinward from said second passage.
 4. The recuperative heat exchanger ofclaim 1 wherein said first and second convoluted fins have surfaceenhancements.
 5. The recuperative heat exchanger of claim 4 wherein saidsurface enhancements are fin louvers.
 6. The recuperative heat exchangerof claim 1 wherein the first and second convoluted fins are bonded toopposite sides of a wall that separates the first passage from thesecond passage.
 7. The recuperative heat exchanger of claim 6 whereinsaid fins are only bonded to said wall to allow for thermal expansionrelative to other portions of said housing.
 8. The recuperative heatexchanger of claim 1 further comprising a cylindrical water-gas shiftreactor extending centrally through said housing at a location radiallyinward from said first and second passages.
 9. A recuperative heatexchanger for use in a fuel processing system, the heat exchangertransferring heat from a fluid flow at one stage of a fuel processingoperation to said fluid flow at another stage of the fuel processingoperation, the heat exchanger comprising: a cylindrical wall; a firstconvoluted fin bonded to a radially inwardly facing surface of saidwall, the convolutions of the first convoluted fin extending axially todirect said fluid flow through the heat exchanger; and a secondconvoluted fin bonded to a radially outwardly facing surface of saidwall, the convolutions of the second convoluted fin extending axially todirect said fluid flow through the heat exchanger.
 10. The recuperativeheat exchanger of claim 9 wherein said fins have surface enhancements.11. The recuperative heat exchanger of claim 10 wherein said surfaceenhancements are fin louvers.
 12. The recuperative heat exchanger ofclaim 9 further comprising a cylindrical water-gas shift reactorextending centrally through said heat exchanger at a location radiallyinward from said first fin.
 13. A fuel processing system comprising: asteam reformer; and a recuperative heat exchanger, the heat exchangerconnected to the steam reformer to direct a fluid flow to the steamreformer, the fluid flow being a steam/fuel feed mix to be reformed insaid steam reformer into a reformate, the steam reformer connected tothe heat exchanger to direct the fluid flow back to the heat exchangerafter it has been reformed into said reformate; the heat exchangercomprising: a cylindrical wall; a first convoluted fin bonded to aradially inwardly facing surface of said wall, the convolutions of thefirst convoluted fin extending axially to direct said fluid flow throughthe heat exchanger; and a second convoluted fin bonded to a radiallyoutwardly facing surface of said wall, the convolutions of the secondconvoluted fin extending axially to direct said fluid flow through theheat exchanger.
 14. The fuel processing system of claim 13 wherein saidfins have surface enhancements.
 15. The fuel processing system of claim13 wherein said surface enhancements are fin louvers.
 16. The fuelprocessing system of claim 13 further comprising a cylindrical water-gasshift reactor extending centrally through said heat exchanger at alocation radially inward from said first fin.
 17. A fuel processingsystem comprising: a steam reformer; and a recuperative heat exchanger,the heat exchanger connected to the steam reformer to direct a fluidflow to the steam reformer, the fluid flow being a steam/fuel feed mixto be reformed in said steam reformer into a reformate, the steamreformer connected to the heat exchanger to direct the fluid flow backto the heat exchanger after it has been reformed into said reformate;the heat exchanger comprising: a housing defining first and secondaxially extending, concentric annular passages in heat transfer relationto each other; a first convoluted fin located in the first passage todirect the fluid flow therethrough; and a second convoluted fin locatedin the second passage to direct the fluid flow therethrough.
 18. Thefuel processing system of claim 17 wherein said housing further definesa first inlet to direct the fluid flow into the first passage at one endof the first passage, a first outlet to direct the fluid flow from thefirst passage at an opposite end of the first passage, a second inlet todirect the fluid flow into the second passage at a location adjacentsaid opposite end, and a second outlet to direct the fluid flow from thesecond passage.
 19. The fuel processing system of claim 17 wherein saidfirst passage is located radially inward from said second passage. 20.The fuel processing system of claim 17 wherein said first and secondconvoluted fins have surface enhancements.
 21. The fuel processingsystem of claim 20 wherein said surface enhancements are fin louvers.22. The fuel processing system of claim 17 wherein the first and secondconvoluted fins are bonded to opposite sides of a wall that separatesthe first passage from the second passage.
 23. The fuel processingsystem of claim 22 wherein said fins are only bonded to said wall toallow for thermal expansion relative to other portions of said housing.24. The fuel processing system of claim 17 further comprising acylindrical water-gas shift reactor extending centrally through saidhousing at a location radially inward from said first and secondpassages.